Measurement of the 154Gd(n,γ) cross section and its astrophysical implications

2020

Publication Year

2021-01-21T14:38:07Z

Acceptance in OA@INAF

þÿMeasurement of the 154Gd(n,³) cross section and its astrophysical implications

Title

Mazzone, A.; CRISTALLO, Sergio; Aberle, O.; Alaerts, G.; Alcayne, V.; et al.

Authors

10.1016/j.physletb.2020.135405

DOI

http://hdl.handle.net/20.500.12386/29924

Handle

PHYSICS LETTERS. SECTION B

Journal

804

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

154

Gd(n,

γ

)

cross

section

and

its

astrophysical

implications

A. Mazzone

a

,

b

,

S. Cristallo

c

,

d

,

O. Aberle

e

,

G. Alaerts

f

,

V. Alcayne

g

,

S. Amaducci

h

,

i

,

J. Andrzejewski

j

,

L. Audouin

k

,

V. Babiano-Suarez

l

,

M. Bacak

e

,

m

,

n

,

M. Barbagallo

e

,

a

,

V. Bécares

g

,

F. Beˇcváˇr

o

,

G. Bellia

h

,

i

,

E. Berthoumieux

n

,

J. Billowes

p

,

D. Bosnar

q

,

A.S. Brown

r

,

M. Busso

c

,

s

,

M. Caamaño

t

,

L. Caballero

l

,

M. Calviani

e

,

F. Calviño

u

,

D. Cano-Ott

g

,

A. Casanovas

u

,

D.M. Castelluccio

v

,

w

,

F. Cerutti

e

,

Y.H. Chen

k

,

E. Chiaveri

p

,

x

,

e

,

G. Clai

v

,

w

,

N. Colonna

a

,

G.P. Cortés

u

,

M.A. Cortés-Giraldo

x

,

L. Cosentino

h

,

L.A. Damone

a

,

y

,

M. Diakaki

z

,

M. Dietz

aa

,

C. Domingo-Pardo

l

,

R. Dressler

ab

,

E. Dupont

n

,

I. Durán

t

,

Z. Eleme

ac

,

B. Fernández-Domíngez

t

,

A. Ferrari

e

,

I. Ferro-Gonçalves

ad

,

P. Finocchiaro

h

,

V. Furman

ae

,

R. Garg

aa

,

A. Gawlik

j

,

S. Gilardoni

e

,

T. Glodariu

af

,

K. Göbel

ag

,

E. González-Romero

g

,

C. Guerrero

x

,

F. Gunsing

n

,

S. Heinitz

ab

,

J. Heyse

f

,

D.G. Jenkins

r

,

E. Jericha

m

,

Y. Kadi

e

,

F. Käppeler

ah

,

A. Kimura

ai

,

N. Kivel

ab

,

M. Kokkoris

z

,

Y. Kopatch

ae

,

S. Kopecky

f

,

M. Krtiˇcka

o

,

D. Kurtulgil

ag

,

I. Ladarescu

l

,

C. Lederer-Woods

aa

,

J. Lerendegui-Marco

x

,

S. Lo Meo

v

,

w

,

S.-J. Lonsdale

aa

,

D. Macina

e

,

A. Manna

v

,

aj

,

T. Martínez

g

,

A. Masi

e

,

C. Massimi

v

,

aj

,∗

,

P.F. Mastinu

ak

,

M. Mastromarco

e

,

p

,

F. Matteucci

al

,

am

,

E. Maugeri

ab

,

E. Mendoza

g

,

A. Mengoni

v

,

w

,

V. Michalopoulou

z

,

P.M. Milazzo

al

,

F. Mingrone

e

,

R. Mucciola

v

,

aj

,

A. Musumarra

h

,

i

,

A. Negret

af

,

R. Nolte

an

,

F. Ogállar

ao

,

A. Oprea

af

,

N. Patronis

ac

,

A. Pavlik

ap

,

J. Perkowski

j

,

L. Piersanti

c

,

d

,

I. Porras

ao

,

J. Praena

ao

,

J.M. Quesada

x

,

D. Radeck

an

,

D. Ramos Doval

k

,

R. Reifarth

ag

,

D. Rochman

ab

,

C. Rubbia

e

,

M. Sabaté-Gilarte

x

,

e

,

A. Saxena

aq

,

P. Schillebeeckx

f

,

D. Schumann

ab

,

A.G. Smith

p

,

N. Sosnin

p

,

A. Stamatopoulos

z

,

G. Tagliente

a

,

J.L. Tain

l

,

Z. Talip

ab

,

A.E. Tarifeño-Saldivia

u

,

L. Tassan-Got

e

,

z

,

k

,

P. Torres-Sánchez

ao

,

A. Tsinganis

e

,

J. Ulrich

ab

,

S. Urlass

e

,

ar

,

S. Valenta

o

,

G. Vannini

v

,

aj

,

V. Variale

a

,

P. Vaz

ad

,

A. Ventura

v

,

D. Vescovi

c

,

as

,

d

,

V. Vlachoudis

e

,

R. Vlastou

z

,

A. Wallner

at

,

P.J. Woods

aa

,

R. Wynants

f

,

T.J. Wright

p

,

P. Žugec

q

a_{IstitutoNazionalediFisicaNucleare,Bari,Italy} b_{ConsiglioNazionaledelleRicerche,Bari,Italy} c_{IstitutoNazionalediFisicaNazionale,Perugia,Italy}

d_{IstitutoNazionalediAstrofisica- OsservatorioAstronomicod’Abruzzo,Italy} e_{EuropeanOrganisationforNuclearResearch(CERN),Switzerland}

f_{EuropeanCommission,JointResearchCentre,Geel,Retieseweg111,B-2440Geel,Belgium} g_{CentrodeInvestigacionesEnergéticasMedioambientalesyTecnológicas(CIEMAT),Spain} h_{INFNLaboratoriNazionalidelSud,Catania,Italy}

i_{DipartimentodiFisicaeAstronomia,UniversitàdiCatania,Italy} j_{UniversityofLodz,Poland}

k_{IPN,CNRS-IN2P3,Univ.Paris-Sud,UniversitéParis-Saclay,F-91406OrsayCedex,France} l_{InstitutodeFísicaCorpuscular,CSIC- UniversidaddeValencia,Spain}

m_{TechnischeUniversitätWien,Austria}

n_{CEASaclay,Irfu,UniversitéParis-Saclay,Gif-sur-Yvette,France} o_{CharlesUniversity,Prague,CzechRepublic}

p_{UniversityofManchester,UnitedKingdom}

q_{DepartmentofPhysics,FacultyofScience,UniversityofZagreb,Croatia}

*

Correspondingauthorat:DipartimentodiFisicaeAstronomia,UniversitàdiBologna,Italy.

E-mailaddress:Cristian.Massimi@bo.infn.it(C. Massimi).

https://doi.org/10.1016/j.physletb.2020.135405

0370-2693/©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

2 A. Mazzone et al. / Physics Letters B 804 (2020) 135405

r_{UniversityofYork,UnitedKingdom}

s_{DipartimentodiFisicaeGeologia,UniversitàdiPerugia,Italy} t_{UniversityofSantiagodeCompostela,Spain}

u_{UniversitatPolitècnicadeCatalunya,Spain}

v_{IstitutoNazionalediFisicaNucleare,SezionediBologna,Italy}

w_{Agenzianazionaleperlenuovetecnologie,l’energiaelosviluppoeconomicosostenibile(ENEA),Bologna,Italy} x_{UniversidaddeSevilla,Spain}

y_{DipartimentodiFisica,UniversitàdegliStudidiBari,Italy} z_{NationalTechnicalUniversityofAthens,Greece}

aa_{SchoolofPhysicsandAstronomy,UniversityofEdinburgh,UnitedKingdom} ab_{PaulScherrerInstitut(PSI),Villigen,Switzerland}

ac_{UniversityofIoannina,Greece}

ad_{InstitutoSuperiorTécnico,Lisbon,Portugal}

ae_{JointInstituteforNuclearResearch(JINR),Dubna,Russia}

af_{HoriaHulubeiNationalInstituteofPhysicsandNuclearEngineering(IFIN-HH),Bucharest,Magurele,Romania} ag_{GoetheUniversityFrankfurt,Germany}

ah_{KarlsruheInstituteofTechnology,CampusNorth,IKP,76021Karlsruhe,Germany} ai_{JapanAtomicEnergyAgency(JAEA),Tokai-mura,Japan}

aj_{DipartimentodiFisicaeAstronomia,UniversitàdiBologna,Italy} ak

IstitutoNazionalediFisicaNucleare,SezionediLegnaro,Italy al_{IstitutoNazionalediFisicaNazionale,Trieste,Italy} am_{DipartimentodiFisica,UniversitàdiTrieste,Italy}

an_{Physikalisch-TechnischeBundesanstalt(PTB),Bundesallee100,38116Braunschweig,Germany} ao_{UniversityofGranada,Spain}

ap_{UniversityofVienna,FacultyofPhysics,Vienna,Austria} aq_{BhabhaAtomicResearchCentre(BARC),India} ar_{Helmholtz-ZentrumDresden-Rossendorf,Germany} as_{GranSassoScienceInstitute,L’Aquila,Italy} at_{AustralianNationalUniversity,Canberra,Australia}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received4February2020

Receivedinrevisedform30March2020 Accepted31March2020

Availableonline3April2020 Editor:D.F.Geesaman

Keywords:

sprocess

154_Gd

Neutrontimeofflight n_TOF

Theneutroncapturecrosssectionof154Gdwasmeasuredfrom1eVto300keVintheexperimentalarea located185mfromtheCERNn_TOFneutronspallationsource,usingametallicsampleofgadolinium, enrichedto67% in154_{Gd.Thecapture measurement,performedwithfourC}

6D6 scintillationdetectors,

hasbeencomplementedbyatransmissionmeasurementperformedattheGELINAtime-of-flightfacility (JRC-Geel), thus minimising the uncertainty related to sample composition. An accurate Maxwellian averagedcapturecrosssection(MACS)wasdeducedoverthetemperaturerangeofinterestforsprocess nucleosynthesismodelling.Wereportavalueof880(50)mbfortheMACSatkT=30 keV,significantly lowercomparedtovaluesavailableinliterature.Thenewadopted154_Gd(n,

_γ

_{)crosssectionreducesthe}

discrepancy betweenobserved andcalculated solar s-onlyisotopicabundancespredicted bys-process nucleosynthesismodels.

©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Alltheelementsheavierthanthoseintheirongroup are pro-ducedby a sequence ofneutron capture reactions and

β

decays takingplaceina hotstellar environment,during differentphases ofstellarevolution.Thetwomainprocessesinvolvedaretheslow (s)andtherapid(r)neutroncaptureprocesses.Thesprocess[1,2] owesits nametotheneutron-capture timescale,whichallows

β

decaytooccur betweenconsecutivecaptureevents.Consequently, aseriesofthesereactionsproducestableisotopesbymovingalong the

β

-stability valley.On theother hand,whenthe neutron den-sitiesarehighenough[3], theneutroncapturesequenceismuch fasterthanthe

β

decaysandthepath,therprocesspath,can pro-ceedtoward many short-lived isotopes, approaching the neutron dripline.

Mostnuclei receive a contributionfrom both the s andthe r processes(seee.g.[4]).However,afewisotopescannotreceiveany contributionfromtherprocess becausetheyareshieldedagainst

β

decays by stable isobars and for this reason are called s-only isotopes. This is the case of the two gadolinium isotopes 152Gd and 154Gd which are shielded against the

β

-decay chains from the r-process region by their stable samariumisobars, as shown inFig.1.Tobeprecise,152_{Gdmayreceiveanadditional}

contribu-tionfromthep process,whichproceedsvia photo-disintegration.

Fig. 1. The yellowlinerepresentsthemains-processpathintheSm-Eu-Gdregion; thes-onlyisotopesarehighlightedwithayellowdashedbox.Stableelementsare inblack,β+,β−andβ+radioisotopesareinorange,blueandred,respectively.

Theamountofthep-processcontributiontothe152Gdabundance isstillfarfrombeingpreciselydetermined(see[5] andreferences therein),whileaminorp-processcontributiontothe154_Gd

abun-dancecannotbeexcludedaswell.

The almostpure s-processoriginof154Gd(asforother s-only isotopes),makesitscapturecrosssectioncrucialfortestingvarious stellar models aimingatunderstanding s process nucleosynthesis in AsymptoticGiant Branch(AGB) stars,themostimportant

stel-larsiteforthesprocess.Inparticular,relevanthintsontheshape andextensionofthemainsprocessneutronsource,theso-called

13_{C pocket [6], can be derived. Recently, several studies [7,8,4]}

have been conducted analyzing the solar s process abundances in the framework of a Galactic Chemical Evolution (GCE) model toinvestigatetheeffectofdifferentinternalstructures ofthe 13C pocket,whichmayaffecttheefficiencyofthe13C(

α

,n)16Oreaction. In addition, Trippella and Collaborators [9] carried out a similar analysis based on single stellar models. Cristallo et al. [8] and Prantzos et al. [4] have obtained an under-production (-30%) of the154Gd1 s-onlynucleuscomparedto thes-only isotope150Sm, whichisan un-branched isotope,usually assumedasareference for the s process flow. This result is at odds with observations. The authors ofref. [8], have suggested that part ofsuch a devi-ationcould be connectedto uncertainties intheadopted nuclear physicsinputs,takenfromtheKarlsruheAstrophysicalDatabaseof NucleosynthesisinStars(KADoNiS) version0.3 [10],includingthe neutroncapture cross section of 154_{Gd. In the work by Trippella}

etal. [9],problemsforthesprocessproductionofGdwerefound aswell,although inthiscase,thediscrepancybetweenthe abun-dancesof150_Smand154_{Gdislesspronounced.Theneedforanew} 154_Gd(n,

_γ

_{)measurementwasunderlinedbytheunreasonable}

pre-dictionforthe over-productionsof152Gdand154Gdwithrespect to their solar abundances. In particular, the ratio 154Gd/152Gd), turnedout tobelower thanunity, whileitisthoughtthat 152Gd should exhibit a higher p-process contribution as compared to

154_{Gd [9]. In addition,} 154_{Gd was found to be produced}

insuffi-cientlycomparedtothelighters-only148Sm and142Nd,produced mostlybythesprocessandpossiblypartlybythepprocess.

Theclose correlationbetweenstellar abundances andneutron capturecross sections calls foran accurate determination of the

154_Gd(n,

_γ

_{) crosssection. In addition,thereduction ofthe}

uncer-taintyrelatedto nuclearphysics inputscouldrule outone ofthe possiblecausesofpresentdiscrepancies betweenobservationand modelpredictions oftheabundances. Infact,refinedstellar mod-els requirea full set ofMaxwellian AveragedCapture Cross Sec-tion (MACS) forthermal energies in the range kT

=

8

−

30 keV. In the case of 154_{Gd, 80% of the MACS at kT}

₌

_{8 keV is}

deter-mined bythe capturecrosssection in theneutron energyregion between2

.

7

−

300 keV- hereafter werefertothisenergyregion asUnresolvedResonanceRegion(URR).At

kT

=

30 keV,theMACS dependsalmost entirelyon the cross section in the URR. In this energyregion, threetime-of-flight 154_Gd(n,

_γ

_{) cross section}

mea-surementsarereportedinliterature,byShorinetal.[11],Beerand Macklin[12] andWisshaketal.[13].Theyall covertheURR and therespectiveMACSexhibit largedifferencesandat

kT

=

30 keV, theyrangefrom878(27)mbto1278(102)mb.Therefore,thedata presentintheliterature,sofar,arenotconclusiveenoughto con-strainstellarmodelcalculations.

The large spread in the available experimental data could be relatedtothecorrectionsforisotopicimpuritieswhichare neces-sarilyapplied in thedata analysis. Inparticular, the poor knowl-edge oftheir cross sections,given the low naturalabundance of

154_{Gd(2.18%).Differentdiscrepanciesmayberelatedto:}

_{i) the}

de-tectorsusedin thepast experiments,which insome cases could havesuffered from highneutron sensitivity;ii) the experimental determinationofthe neutronflux,whichmighthavebeenbiased insomeprevious measurements;iii) the qualityofoxidesamples andtheneedofcanningforthecontainertoavoidlossofmaterial. Thepresentmeasurementreducedtheimpactoftheselimiting factors,byusingthewell-established,lowneutron-sensitivityC6D6

detectors[14], combined toa self-sustaining metallic sample

en-1 _{Inthefollowing,anindividualisotopeor/anditsabundanceisimplyindicated}

bythesymbol(e.g.152_{Gd),tosimplifythenotation.}

richedin154_{Gd,andexploitingtheresultsoftherecent}155_Gd(n,

_γ

₎

measurementperformedatn_TOF[15].Moreover,thegadolinium sample was characterisedby a transmission measurement atthe neutrontime-of-flightfacilityGELINAatEC-JRC-Geel(Belgium).

2. Measurements

Theneutroncapturecrosssectionmeasurementwasperformed attheneutron time-of-flightfacilityn_TOFatCERN.In this facil-ity, neutrons are produced by spallation reactions induced on a leadtargetby20GeV/cprotonsfromtheCERNProtonSynchrotron (PS),whichprovidesatotalof2

×

1015neutrons/pulse,generated by a 7

×

1012 _{protons/pulse primary beam. The initially fast}

neu-tronsaremoderatedandthencollimatedthroughtwoflightpaths of differentlengths. The presentmeasurement was performedat the experimental area located 185 m from the spallation target. Thislongflight-path,combinedwiththe7nswidthoftheproton bunches from the PS, results in a high energy resolution rang-ing from



E

/

E

=

3

×

10−4 at 1 eV to



E

/

E

=

3

×

10−3 at 100 keV(calculated usingtheFWHMof theresolutionfunction [16]). Theneutroncaptureeventswereobservedviathedetectionofthe prompt

γ

-raycascadefrom155Gdexcitedstates.FourC6D6

detec-tors, modified forminimizing their neutronsensitivity [14],were arrangedat125◦ relativetotheneutronbeamdirectionandabout 10cmupstreamfromthegadoliniumsampleposition.This config-urationminimisedtheeffectofanisotropicemissionof

γ

cascades whilereducingthebackgroundfromin-beamphotonsscatteredby the sample. The Total Energy detectors (see [17] and references therein) were used in combination with Pulse Height Weighting Technique(PHWT)[18,17].

The sample consisted of 0.263 g of metallic gadolinium en-richedat66.78% in 154Gd. Themaincontaminant, i.e.155Gd,was declared by the sample providerat 17.52%. Adetailed resonance analysisofthecapturedata,basedontherecentresultsobtained bytimeofflighton155Gd(n,

γ

)atn_TOF,allowedustoestimatea contentof155Gdequalto20.2%.Thishighervaluewas confirmed byatransmissionmeasurementonthesamesamplecarriedoutat GELINA.

AAusample ofthesamediameterwas usedtonormalisethe measured yield, by applying the saturated resonance technique [19]. Also, two other samples with the same diameter of 3 cm wereused.Aleadsampleenabledtheestimateofthebackground, whileanaturalgadoliniumsampleallowedtoassignobserved res-onancestothecorrectgadoliniumisotope,besidesconfirming the isotopiccontentof154Gdintheenrichedsample.

Asmentioned above,thegadoliniumsample was further stud-ied through the transmissionmeasurement ata 10-m station of the GELINA facility. The transmission, T , which was experimen-tally obtained fromthe ratio of Li-glass spectra resulting froma sample-in anda sample-out measurement,is relatedto thetotal crosssection

σ

tot bytheequation:

T(En

)

=

e−nσtot(En)

,

(1)

where

n

= (

1

.

431

±

0

.

006

)

×

10−4 _{atoms/bdenotestheareal}

den-sity ofthegadolinium sample. GELINAis particularlysuitable for high-resolution transmission measurement, because of its time characteristic and the small dimensions of the neutron produc-ing target.Forthisexperiment,the neutronbeamwascollimated to a diameterof 10 mm atthe sample position andfilters were placednearthesampletoabsorbslowneutronsfromtheprevious neutron-burst and to continuously monitor the background. The neutronbeampassing through thesample was detected bya 6.4 mmthickand76mmwideLi-glassscintillatorenrichedto95%in

6_{Li.Thedetectorwasplacedat10.86mfromtheneutron}

4 A. Mazzone et al. / Physics Letters B 804 (2020) 135405

3. Dataanalysisandresults

The experimental capture yield Yc, i.e. the probability for an incident neutron to be captured in the sample, can be deduced from the measured count rate, corrected for the detection effi-ciency of captureevents. By applyingthe PHWT, the count rate, Cw, is weighted in order to make the detection efficiency inde-pendent of the cascade path and

γ

multiplicity. The weighting functionswerecalculatedsimulatingtheresponseofthefull appa-ratusbyaGEANT4[20] Monte-Carlosimulation.Thecaptureyield canbewrittenas:

Y

(

En

)

=

N

Cw

(E

n

)

−

Bw

(E

n

)

(E

n

)

(2)

where

N is

a normalisationfactor, Bw istheweighted count rate representingthebackgroundand



istheneutronfluximpinging on thesample. The neutron energy

E

n was determined fromthe measured timeofflight usingan effectiveflight pathdetermined fromwell-knownlowenergyresonancesinAu[21].

The normalisation factor groups together geometrical factors, such as the area of thesample andits beam-interceptionfactor, thesolid angle subtendedby the captureandflux monitors, and the detection efficiency. It was obtained with the saturated res-onance technique applied to the 4.9-eV resonance in197_{Au. This}

normalisation,basedontheAucapturedata,waswithin1.3% con-sistent withthe normalisation derived froma fit to the capture datausingtheparametersfromthetransmissiondata.

The background,Bw, includesdifferentcontributions:

i)

ambi-ent background, which was determined from a measurement in absence of the neutron beam; ii) the sample-independent back-ground(alsoreferredtoasemptybackground),duetotheneutron beam, which was estimated from a measurement with neutron beamimpinging on an empty sample;iii) the sample-dependent background,eitherduetosample-scatteredneutrons(subsequently capturedin theenvironmental materialandgenerating

γ

-rays in the experimental area) ordue to

γ

-rays produced at the spalla-tiontargetandreachingtheexperimentalarea,wheretheycanbe scatteredby thesample intothe detectors.Thisthird component wasestimatedbya measurementwithaleadsample inthe neu-tron beam. Previous measurements and Monte-Carlo simulations showedthat thecontributionofthein-beam

γ

-raybackgroundis relevantonlyinalimitedenergywindowof1-100keV [16]. There-fore, it was possible to disentangle the two sample-dependent backgroundcomponents inthe time-of-flightspectrum measured withthe lead andwiththe gadoliniumsamples, properlyscaled. The neutron background was scaled forthe elastic cross section and the areal density of the gadolinium andlead sample, while the

γ

backgroundwas scaled forthe effectiveatomicnumber of gadolinium and lead samples. The same procedure for the esti-mation of thetotal background was adopted inthe study ofthe

197_Au(n,

_γ

_{)crosssection.Inparticular,intheenergyregionfrom5}

keVto500keV,wherethiscrosssection isverywell-known (and above200keV,consideredasastandard)thecapturecrosssection fromthisstudyresultedtobeingoodagreementwithevaluations. In Fig. 2the weighted number ofcounts, registered withthe

154_{Gdsample, are shown together with the sample-independent}

(empty)backgroundandthe sample-dependentbackground com-ponents.Theempty-samplebackgroundcomponentdominatesthe total backgroundover the energyregion of interest,whereas the sample-dependentbackgroundcontributesby lessthan 4% tothe totalbackground.The absolutemagnitudeofthe backgroundwas additionallyverifiedwithdedicatedrunsusingblackresonance fil-ters [17] andafairagreementwasfound.Intheregionbetween5 and300keV,thesignal-to-totalbackgroundratiois2.5.Although the total background level is significant, its uncertainty is small

Fig. 2. Weighted spectrummeasuredwiththe154_{Gdsamplecomparedtothe}

back-groundmeasured with an empty-sampleholderand the one estimatedfrom a measurementwithaleadsample.Thelastspectrumwasobtainedbyscalingthe in-beam

γ

-raybackgroundandthecomponentduetotheneutronsscatteredby thesample,seetextfordetails.

Fig. 3. Measured captureyieldandtransmissionofforn+154_{Gd.Experimentaldata}

areshownbyredsymbols,then_TOFR-matrixfitinblueandtheyieldcalculated fromENDF/B-VIII.0parametersingreen,bothincontinuouslines.

since thebackground isdominatedby theempty-sample compo-nent,whichisknownwithin1%.

Theneutronfluxwasevaluatedwithadedicatedmeasurement campaignusingdifferentdetectionsystemsandwithneutroncross section standards [22]. The estimated systematic uncertainty on the flux determination was within 1% below 3 keV [22] and of 3.5% upto300keV.

Intheenergyregionupto2.7keV- hereafterreferredtoas Re-solved Resonance Region(RRR) -,the experimental capture yield was analysed with the Bayesian R-matrix analysis code SAMMY [23].The code can manage experimental effects such asDoppler andresolutionbroadening,self-shieldingandmultipleinteractions of neutrons in the sample. Sizable discrepancies were found for some neutronresonances compared to the yields obtained using theENDF/B-VIII.0evaluationdataset[24].Thesedifferenceswere furtherconfirmedbythetransmissiondata.Anexampleisshown in Fig. 3, where the present results of resonance shape analysis arecomparedtotheexpectedvaluesobtainedusingresonance pa-rametersfromtheevaluateddataset.Upto2.75keV,weanalysed 156 resonances and 3 new resonances were found at 183.17(2), 197.65(1) and 376.26(1) eV, respectively. The statistical analysis of resonance parameters (similar to ref. [15]) yielded a neutron strength function S0

=

2

.

78

(

33

)

×

10−4, an average levelspacing

Fig. 4.154_{Gd(n,γ) crosssectionfromthepresentstudycomparedtoprevious}

mea-surements(colouredsymbols)andevaluation(continuousline).TheHFcalculation hasbeennormalisedbyafactor0.737(seetext).

Abovethisenergyregion, theexperimental resolution became toolowtoresolveindividualresonancesandanaveragedcross sec-tionwas determinedintheneutronenergyrangefrom2.7keVto 300keV. The captureyield was correctedfor multiple scattering and self-shielding through Monte-Carlo simulations as described inref. [25].Theresultingcorrectionforthetwoeffectswas lower than2%.

Thecontributionofthecontaminantspresentinthemeasured sampleswastakenintoaccountintheexperimentaldataanalysis. In particular, for the main contaminant 155_{Gd, the cross section}

was assumed from the previous n_TOF measurement on 155Gd. In Fig. 4, the capture cross section extracted from this study is comparedtothedataofShorinetal. [11],BeerandMacklin [12], Wisshaketal. [13] andtheENDF/B-VIIIevaluation.IntheURR,the presentdatafairly agreewiththe databy BeerandMacklin[12] andthey are substantially lower than thedata reported by Wis-shak etal.[13] and Shorin etal.[11].This comparisonseems to indicatethat the resultsobtained fromthiswork andby Beeret al.withthesametechnique areinfairagreement,whilethey are inconsistentwiththeresultsbyWisshaketal.andShorinetal. Al-thoughtheselatterdatacomefromasimilarexperimental setup, whereneutronareproducedviathe7Li(n,p)reaction,theneutron flux is normalised to 197_Au(n,

_γ

_{) and the capture eventsare}

de-tectedwith large-volumedetectors, their resultsare inconsistent. AsdiscussedaboveintheIntroduction,thediscrepanciesmightbe relatedtoa problemwiththeneutronsensitivityofthedetectors ortotheexperimentaldeterminationoftheneutronflux.

MACSasa function ofthe thermalenergykT were calculated from the present capture data in the RRR and in the URR. The crosssectionintheenergyregionabovethisrange(i.e.above300 keV)was takenintoaccount by calculationsperformedusingthe HauserFeshbach(HF)statisticalmodeltheory,asimplementedin theTALYScode [26]. Theaverage resonanceparameters,obtained inthepresentanalysisoftheRRR,were constrainedtobe repro-ducedby the calculationsby adjusting the level densityand the

γ

-raystrengthfunction.Anoverallnormalisationbyafactor0.737 oftheresultingcapturecrosssectionwas stillnecessaryto repro-ducethepresentexperimentalMACSat

kT

=

30 keV.

Theuncertainty onthe MACS takesinto account the uncorre-lateduncertaintyattributabletocountingstatisticsandsystematic uncertainties.Theuncertaintycomponentsoriginatefromthe nor-malisationofthecapturedataandthePHWT(1.3%),theshapeof theneutronflux(1% below3keVand3.5% above)andthe subtrac-tionofthe background(on average2%, depending on theenergy region.). Anotherminor uncertainty is associated withthe align-mentofthesampleanditsgeometricalshape.Asaresult,overthe thermalenergyrangeofkT

=

5

−

100 keV,theuncertaintyonthe MACSrangesbetween5and7%.InTable1,thepresentMACSare

Table 1

MaxwellianAveragedCaptureCross(inmb)calculatedfor then_TOFdataintheenergyrangekT=5−100 keV.The valuesarecomparedwiththosereportedintheliterature byKADoNiS0.3andKADoNiS1.0.

Energy n_TOF KADoNiS KADoNiS

(keV) 0.3 1.0 5 2160(90) 2801 2947(190) 8 1820(80) – 2195(87) 10 1590(80) 1863 1924(61) 15 1270(80) 1477 1537(33) 20 1080(60) 1258 1326(24) 25 970(50) 1124 1188(19) 30 880(50) 1028(12) 1088(16) 40 770(40) 898 950(14) 50 690(40) 810 856(12) 60 640(40) 745 786(12) 80 560(30) 653 690(15) 100 510(30) 591 626(19)

reportedforthethermalenergygridproposedbyKADoNiS[10,27]. In the whole energy range,the MACS valuesfrom our measure-mentaresignificantlylowerthanKADoNiS.Itisinterestingtonote thatthedisagreementworsenedwiththenewversionofthe eval-uation,thedeviationbeingbetween10% and20%.

4. Astrophysicalimplications

As discussed above, a new determination of the 154Gd neu-troncapturecrosssectionwasmotivatedbyadiscrepancybetween stellar models and observations, as highlighted by [8] and con-firmed by [4], where a lower theoretical 154Gd/150Sm ratio with respect to that measured in the Sun (and derived forthe early-solarsystem)wasfound.Infact,theoreticalvalues,whichinclude yields fromtheAsymptotic GiantBranch(AGB) phaseoflow and intermediate massstars (takenfromthe FRUITYdatabaseofAGB starnucleosynthesis[28,29]),show an under-productionof154Gd withrespectto150_Sm:154_Gd/150_{Sm=0.70.Asaconsequence,one}

canarguethatareasonforthedisagreementshouldbeattributed toproblemsinthecrosssectionof154_Gditself.

Theuseofthepresentcrosssectionleadstoanincreaseofthe theoreticalsolar154_{Gdabundanceby10%onaverage.}2 _The differ-ence in the 154_{Gd surface abundances is lower than the change}

of the neutroncapture crosssections (on average 15% compared toKaDoNiS0.3).Thisisbecausethe154_{Gdproduction/destruction}

strongly depends on the branching at 154Eu, which is an unsta-ble isotope (its decay lifetime in the terrestrial condition is 8.6 y).Thisbranchingisby-passedwhenthemajorneutronsourcein AGB stars(the 13C(

α

,n)16Oreaction) isactivated,duetoits short lifetime forthe timescale characterizing neutroncaptures in this regime.Thesituationmaybedifferentduringthermalpulseswhen thehighertemperaturecanefficientlyactivatethe22_Ne(

_α

_,n)25_Mg

neutronsource(which producesa definitelyhigherneutronflux). In such a case, the neutron capturechannel is competitive com-pared to the

β

decay channel and, as a consequence, the main s-processpath may by-pass 154Gd. The delicate balance between neutron captures on 154_Gdand

_β

_{decays from} 154_{Eu determines}

thefinalabundanceof154Gd.

In summary,the presentexperimental value leads toa better agreementbetweenmodelcalculationsandobservations,although itisnotabletocompletelyremovethemismatch.In2009,Lodders andcollaborators [30] estimatedtheuncertaintyinthe determina-tion of the solar gadolinium3 abundance to be

±

15% (and

±

5%

2

NotethatonlyalimitednumberofAGBmodelshavebeeninvestigatedwith thepresentcrosssection.TheevaluationoftheeffectinafullGCEmodelwillbe publishedseparately.

6 A. Mazzone et al. / Physics Letters B 804 (2020) 135405

forsamarium).Theadoptionofthepresent154_{Gdneutroncapture}

crosssection,eventuallyleadstoanew154Gd/150Sm ratioof0.77 inFRUITYmodels.Whentakingintoconsiderationthelowerlimit of the present neutron capture cross section, we obtain 154_Gd/ 150_Sm

_

_{0.81. As a consequence, the ratios obtained in FRUITY}

modelsarenowcompatiblewiththeobservedabundances,within observationaluncertainties(

±

20%).

Whenthepresentcrosssectionisusedinthemodelsby Trip-pellaetal. [9,31],itisinterestingtonoticethatwhilethe discrep-ancywithrespectto142_Nd,148_Smand152_{Gdiscompletelyerased}

(duetothelargerproductionof154_{Gd),atthesametime, the}

ra-tio154Gd/150Smattainsavalue(1.15)consistentwithobservations, within uncertainties. In general, it appears that the approach in ref. [31] produces a flatter distribution of s process isotopes, al-though this was obtained in post-process computations and not infullstellarmodels.Therefore,aclearsuggestionemergingfrom thepresent154_Gd(n,

_γ

_{)crosssectionmeasurementisthatsomeof}

theremainingmodelambiguitiesmightbesolvedbyamergingof themixingapproachespresentedinFRUITYandref. [9],something thatisinanadvancedstageofimplementation[32].

Anotherimportant outcome fromthiscombined experimental and theoretical study is related to the 154Gd abundance, which largelydependsonthebranchingat154_{Eu.Forthisisotope,no}

ex-perimental dataare available for both the neutron capturecross section and the temperature-dependent

β

decay rate. Therefore, sprocess calculationsarebased onpurely theoreticalestimations (seeref. [33] andref. [34],respectively). Alower 154_Eu(n,

_γ

_{) cross}

section or a faster 154Eu(

β

−)154Gddecay would lead to a larger

154_{Gdsurface abundance withrespect to} 150_{Sm (andvice-versa).}

Therefore,thepresentresultsuggeststhatadditionaleffortsshould be spent inthisdirectionfrom theexperimental side, to provide experimentalvaluesasdetailedaspossibletostellarmodellers.

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

Acknowledgements

The isotopeused in thisresearch was supplied by theUnited StatesDepartmentofEnergyOfficeofSciencebytheIsotope Pro-gramintheOfficeofNuclearPhysics.

This research was supported by the EUFRAT open access pro-grammeoftheJointResearchCentreatGeel.

References

[1]E.M.Burbidge,G.R.Burbidge,W.A.Fowler,F.Hoyle,Rev.Mod.Phys.29(1957) 547–650.

[2]A.G.W.Cameron,Publ.Astron.Soc.Pac.69(1957)201.

[3]F.-K.Thielemann,etal.,Prog.Part.Nucl.Phys.66 (2)(2011)346.

[4]N.Prantzos,etal.,Mon.Not.R.Astron.Soc.491 (2)(2019)1832.

[5]C.Travaglio,etal.,Astrophys.J.854 (1)(2018)18.

[6]R.Gallino,C.Arlandini,M.Busso,M.Lugaro,C.Travaglio,O.Straniero,A.Chieffi, M.Limongi,Astrophys.J.497 (1)(1998)388–403.

[7]S.Bisterzo,etal.,Astrophys.J.787 (1)(2014)10.

[8]S.Cristallo,C.Abia,O.Straniero,L.Piersanti,Astrophys.J.801 (1)(2015)53.

[9]O.Trippella,etal.,Astrophys.J.787 (1)(2014)41.

[10] I.Dillmann,M.Heil,F.Käppeler,R.Plag,T.Rauscher,F.-K.Thielemann,AIPConf. Proc.,vol. 819,2006,p. 123,onlineathttp://www.kadonis.org.

[11]V.Shorin,V.Kononov,E.Poletaev,Sov.J.Nucl.Phys.USSR19 (1)(1974)2.

[12]H.Beer,R.Macklin,Astrophys.J.331(1988)1047.

[13]K.Wisshak,etal.,Phys.Rev.C52 (5)(1995)2762.

[14] P. Mastinu,et al.,Tech.Rep.,Cern(n_TOF-PUB-2013-00226/06/2013). [15]M.Mastromarco,etal.,Eur.Phys.J.A55 (1)(2019)9.

[16]C.Guerrero,etal.,Eur.Phys.J.A49 (2)(2013)27.

[17]P.Schillebeeckx,etal.,Nucl.DataSheets113 (12)(2012)3054.

[18]A.Borella,etal.,Nucl.Instrum.MethodsPhys.Res.,Sect.A577 (3)(2007)626.

[19]R.Macklin,J.Gibbons,Phys.Rev.159 (4)(1967)1007.

[20]S.Agostinelli,etal.,Nucl.Instrum.MethodsPhys.Res.,Sect.A506 (3)(2003) 250.

[21]C.Massimi,etal.,J.KoreanPhys.Soc.59(2011)1689.

[22]M.Barbagallo,etal.,Eur.Phys.J.A49 (12)(2013)156.

[23]N.M.Larson,Tech.Rep.,OakRidgeNationalLab.,TN(US),1998.

[24]D.A.Brown,etal.,Nucl.DataSheets148(2018)1.

[25]F.Mingrone,etal.,Phys.Rev.C95(2017)034604.

[26]A.J.Koning,D.Rochman,Nucl.DataSheets113 (12)(2012)2841.

[27]I.Dillmann,R.Plag,F.Käppeler,T.Rauscher,in:Proc.EFNUDAT2009Workshop, 2010,p. 55.

[28]S.Cristallo,etal.,Astrophys.J.Suppl.Ser.197 (2)(2011)17.

[29]S. Cristallo, O. Straniero, L. Piersanti,D. Gobrecht, Astrophys.J. Suppl. Ser. 219 (2)(2015)40.

[30]K.Lodders,H.Palme,H.Gail,JETrümper4(2009)44.

[31]O.Trippella,etal.,Astrophys.J.818 (2)(2016)125.

[32] D. Vescovi,et al.,Tech.Rep.,inpreparation.

[33]T.Rauscher,F.-K.Thielemann,At.DataNucl.DataTables75 (1–2)(2000)1.